The present invention relates to a device for determining position coordinates of a cursor on a tablet by detecting eletromagnetic induction between the cursor coil and the grid-shaped conductors formed in the grid structures of the tablet, and in particular to a cordless type digitizer in which a cable for transmitting electric signals between the cursor and the tablet is eliminated.
As a prior art of this invention, there is a U.S. Pat. No. 4,795,858 entitled "Device for Determining Position Coordinates of Points on a Surface" possessed by the applicant of the present invention.
In this prior art, the cursor has a battery for supplying electric power to electric circuits in the cursor. An alternating current generated by these electric circuits, flows in a cursor coil and generates an electromagnetic field on the tablet. This electromagnetic field induces electric voltages in the grid-shaped conductors in the grid structures.
These induced voltages in the grid-shaped conductors are processed in a processor to determine the position coordinates of the cursor on the tablet.
In this device of the prior art, however, the power consumption in the cursor is fairly large and the battery of the cursor must be recharged or renewed rather frequently.
In some applications of a digitizer, in addtion to the coordinates of a point, an attribute (or attributes) of the point is sometimes necessary. For example, an attribute of a point specifies a colour to display the coordinates of the point on a cathode ray tube display.
In a prior art which is disclosed in a U.S. Pat No. 4,697,050, this attribute is represented by the frequency of the alternating current in the cursor coil. But, in such a design, a variety of tuning circuits must be provided in the cursor for generating a variety of frequencies, and in the signal porcessor for tuning to the frequencies. When a relatively large number of attributes, say 16 different attributes, are to be indentified, this method of frequency distinction will be impractical.